Black is happy to remove the dangerous bishop from the board.
8. c4b3  8... f8e8 A somewhat \"mysterious rook move\" as Nimzowitsch phrased a long time ago, but this is well known to opening theory. The rook can be useful to support a plan with e6-d5 or even d5 immediately. 9. e1g1  9... d7d6 Threatening ...Ng4. 10. h2h3 